Phenolic resin dispersions are commercially important materials that are used in an array of diverse products. Examples include coatings, such as anti-corrosive baking enamels for metals and coatings used as food contact surfaces; adhesives; contact adhesives or tack-building agents in many acrylic, vinyl acrylic, vinyl and rubber latexes; binders for organic and inorganic materials; laminates; moldings; and other uses.
Dispersions, however, are susceptible to losing their homogeneity over time due to phase separation by settling, coalescence, agglomeration, etc. of the dispersed phase. Therefore, storage stability, or the long-term ability of a dispersion to maintain homogeneous, well-mixed dispersed and continuous phases, is a major consideration affecting the practical utility and thus ultimately the salability of a dispersion. As is well known in the art generally (and particularly with respect to phenolic resin dispersions), the addition of any component to a dispersion will have an unpredictable effect on dispersion stability. This is due to the potential impact of the component on many factors, acting both independently and interactively, that affect dispersion stability, including zeta potential (i.e., inter-particle electrostatic attraction or repulsion), particle size, pH, and solution conductivity. Moreover, the use of any stabilizing agent can adversely affect the ultimate properties of the materials made from the dispersion. Therefore, the art has been continually challenged with the development of storage stable phenolic resin dispersions having desirable physical and chemical characteristics.
For example, aqueous phenolic resin dispersions made with gums or cellulosic materials as the dispersing agent were found to exhibit poor storage stability. Moreover, the coatings resulting from such dispersions had unacceptable chemical resistance due to the hydrophilic nature of the dispersing agent. Surfactants, such as phosphate esters, have also been tried with limited success, as surfactant-containing dispersions provide unacceptably poor film properties caused by surface migration of the surfactant.
Stable dispersions of phenolic resin polymers have been achieved using a protective colloid material, such as polyvinyl alcohol (PVA), as a dispersant in U.S. Pat. No. 4,400,229. Likewise, novolac resin dispersions are stabilized with PVA in U.S. Pat. No. 4,788,236. Protective colloids can also be used in systems relying on an organic coupling solvent, as described in U.S. Pat. No. 4,124,554, wherein an aqueous dispersion is made from a pre-formed, solid, substantially water-insoluble, phenolic resin (either a novolac or a resole). Similarly, U.S. Pat. No. 5,200,455 teaches the mixing of (a) a pre-formed, solid, and substantially water-insoluble phenolic resin, (b) water, (c) an organic coupling solvent, and (d) PVA, at a temperature and for a period of time sufficient to form a dispersion of the phenolic resin in water. U.S. Pat. No. 5,939,159 teaches an aqueous dispersion prepared from an acrylic resin, an aromatic epoxy resin and a bisphenol-F product comprising a reaction product of bisphenol F and formaldehyde, with the bisphenol-F product being etherified.
U.S. Pat. Nos. 5,548,015 and 5,552,186 address the problem of making stable dispersions of hydrophilic phenolic resin polymers using PVA as a protective colloid. These patents teach that, while PVA works well with hydrophobic phenolic polymers (e.g., bisphenol-A), it does not form stable dispersions with highly functional, hydrophilic resins (e.g., phenol/formaldehyde resins) that are desirable in coating applications due to their chemical resistance properties. The solution offered in these patents is to incorporate a small proportion of highly hydrophobic etherified bisphenol-A resin into an aqueous solution of the hydrophilic phenolic resin and protective colloid to form a stable dispersion with low volatile organics content. The substitution of less hydrophobic additives (e.g., etherified phenol/formaldehyde) in place of the etherified bisphenol-A is taught not to provide stable dispersions.
Unfortunately, dispersions of hydrophilic polymers made according to the above-noted U.S. Pat. Nos. 5,548,015 and 5,552,186 can contain residual amounts of bisphenol-A, which is currently under increasing scrutiny as a potential health risk, especially when it is present in resin formulations that are ultimately used to coat surfaces, such as cans, that contact food. Therefore, there remains a need in the art for aqueous dispersions of hydrophilic phenolic resins that are essentially free of bisphenol-A, as well as other undesirable impurities, such as xylenols, that are often present to some extent in certain of the aromatic alcohols used as starting materials in the preparation of hydrophobic additives that are used to stabilize dispersions of hydrophilic phenolic resins.